1. Field of the Invention
This invention relates to an objective optical system for an endoscope using a solid state image pickup device as an imaging device.
2. Description of the Prior Art
Solid state image pickup devices comprising a charge transfer device having a self-scanning function, such as a charge coupled device (CCD), and a plurality of light receiving devices positioned in the matrix form on a plate have heretofore been put into practice. The solid state image pickup devices have been reduced to a chip size of several millimeters square.
In view of the above circumstances, an attempt has been made to incorporate the solid state image pickup device into the end portion of an endoscope as disclosed, for example, in Japanese Unexamined Patent Publication No. 55(1980)-54933. FIG. 1 is a schematic view showing the conventional objective optical system comprising the solid state image pickup device for an endoscope. In the conventional objective optical system, a solid state image pickup device 1 and an objective lens assembly Lo are positioned so that a light receiving section 1A of the solid state image pickup device 1 is normal to the optical axis O of the objective lens assembly Lo and coincides with the image formation surface of the objective lens assembly as indicated by the mark X. Though the configuration of the objective optical system is simple, problems arise when the objective lens assembly is used at the end portion of an endoscope. More specifically, since the end portion of the endoscope is inserted into body cavities such as the gullet, the stomach, the intestine and the trachea, it is desired to decrease the outer diameter of the end portion of the endoscope. However, in the configuration of FIG. 1, it is impossible to decrease the outer diameter of the end portion of the endoscope appreciably due to the outer dimension of the solid state image pickup device 1. Further, when the solid state image pickup device 1 is provided with a charge accumulating section 1B for temporarily accumulating signal charges in addition to the light receiving section 1A as shown in FIG. 1, the problem with regard to the outer diameter of the end portion of the endoscope becomes worse. Also, since an illumination light guide, air and water feed pipes, a forceps channel, and the like must be positioned in the endoscope body, the arrangement of the solid state image pickup device 1 as shown in FIG. 1 markedly interferes with design flexibility.
FIG. 2 is a schematic view showing an example of he objective optical system to which the present invention pertains. The problems mentioned above can be eliminated by positioning the solid state image pickup device 1 in the longitudinal direction of the endoscope as shown in FIG. 2. The objective optical system comprises an optical path converting prism 2 for turning the optical path O. In this configuration, it is possible to prevent the end portion of the endoscope from becoming large due to the outer dimension of the solid state image pickup device 1.
In the objective optical system for the endoscope, a wide angle optical system is utilized to widen the field of view. Widening of the field of view is realized by a retrofocus optical system consisting, for example, of a front-stage diverging lens unit LA and a rear-stage converging lens unit LB as shown in FIG. 2. However, in the configuration of FIG. 2, since the optical path converting prism 2 is positioned between the objective lens assembly Lo and the solid state image pickup device 1, it is necessary to increase the back focal length of the optical system. Further, in order to realize the configuration of FIG. 2, the exit pupil of the objective optical system should preferably be positioned on the object side with respect to the exit plane of the rear-stage converging lens unit LB for the purpose of decreasing the outer diameter of the front-stage diverging lens unit LA which contributes to increasing the field of view and decreasing the size of the optical path converting prism 2. It is also desired to minimize the longitudinal dimension of the objective optical system.